1. Field of the Invention
The present invention relates to an optical receiving circuit and an optical communication device respectively applied to an optical network using an optical switch, particularly relates to an optical receiving circuit and an optical communication device that can follow the variation of a level of an input optical signal at high speed.
2. Description of the Related Art
In an optical network using an optical space switch, large throughput of approximately a few Gb/s. to 10 Gb/s. per port is acquired by utilizing large capacity of optical signals. Therefore, a small-sized small-latency network with large capacity can be realized, compared with a case using an electrical switch.
In case the scale of such an optical network is enlarged, the dispersion of output power of an optical transmitter of each input port, the dispersion of the loss of an optical connector every path, the dispersion of the loss of a switching element and the dispersion of the loss of an optical fiber cable pile and the dispersion every path of signal light power input to an optical receiver connected to an output port increases. Therefore, as an optical receiver in an output port may receive signals greatly different in a level when a path is switched by an optical space switch, the optical receiver is required to receive according to difference in a level. It is desired that time for following difference in a level among received signals is approximately a few bits in order to reduce guard time in switching possibly and enhance the efficiency of transmission, however, a high speed optical receiver which has the throughput of a few Gb/s or more and to which a few time slots are allocated to meet difference in a level is not examined so much.
A conventional type optical network such as disclosed in Japanese Patent laid-open publication No. Hei 4-72939, xe2x80x9cPacket switching system using photonic switchxe2x80x9d uses an AC coupled optical receiver. The AC coupled optical receiver has advantages that a dynamic range is large and the duty ratio variation is small and can follow a gentle level variation of approximately 1000 time slots or more. The optical receiver uses an automatically gain controlled (AGC) circuit amplifier to receive signals different in a level. The time constant of automatic gain control (AGC) is set to approximately 1xc3x97103 to 1xc3x97105 times of one time slot so that a dc level is unchanged even for long continuation of same bits.
However, in the conventional type AC coupled optical receiver, AC coupling and an AGC time constant are approximately 1xc3x97103 to 1xc3x97105 times of one time slot. Therefore, when two optical signals having difference in a level are switched by an optical switch, it takes time equal to approximately 1xc3x97103 to 1xc3x97105 times of one time slot until the optical receiver can follow the level of an optical signal switched by the optical switch. That is, as it takes 1000 bits or more, it is difficult to follow large difference in a level between optical signals in a few time slots and receive a new optical signal. In the above patent application, the variation of a level in switching is followed by equalizing switching time with an AGC time constant, however, as it takes time equivalent to 1000 bits or more, an efficient small-latency network with large capacity cannot be realized. On the contrary, when AC coupling and an AGC time constant are approximately ten times of one time slot, there is a problem that allowable transmission codes is limited under the variation of a level in switching followed in time equivalent to approximately 10 bits.
In the meantime, a DC coupled receiver that can receive even when the high speed variation of a level occurs in one time slot has two types, that is, a discrimination level fixed type and an instantaneous response automatic discrimination level control type. As for the discrimination level fixed type DC coupled optical receiver, although the configuration and design of circuits are simple, there is a problem that a dynamic range is small and duty ratio greatly varies depending upon the level of a received signal and the optical receiver is not suitable for practical use. The instantaneous response automatic discrimination level control type DC coupled optical receiver controls every time slot so that the discrimination level of a received signal is a suitable value and has excellent characteristics that the variation of duty ratio is small and that a dynamic range is large. However, it is very difficult to manufacture the device that is operated in a wide band of a few GB/s. or more because of a limit in the operating speed of the following device. Therefore, there is a problem that it is difficult to realize a DC coupled optical receiver operated at the frequency of a few Gb/s or more and suitable for practical use. In case an optical data link cannot transmit and receive data correctly when an optical network is actually configured using the optical data link and an optical switch, the network cannot be realized. In case an optical receiver based upon prior art is used, it is difficult to realize a large-scale small-sized small-latency optical network with large capacity.
The object of the invention is to provide an optical receiving circuit and an optical communication device respectively indispensable to realize a large-scale small-sized small-latency optical network with large capacity, operated at the high speed of a few Gb/s or more and which can follow the variation of the level of an input signal in a few time slots.
An optical receiving circuit according to the present invention is provided with a preamplifier circuit which is connected to a photodetector for receiving an optical signal and to which photo current from the photodetector is input, an output differential amplifier to which the output of the preamplifier circuit is input and a reference voltage generating circuit to which reference voltage generated based upon a mean value per time of voltage output from the preamplifier circuit is input to be referred by the output differential amplifier.
An optical receiving circuit according to another aspect of the present invention is provided with a preamplifier circuit which is connected to a photodetector for receiving an optical signal and to which photo current from the photodetector is input, plural differential amplifiers to which a signal output from the preamplifier circuit is input and which have threshold voltage respectively different, an output voltage holding circuit for holding voltage respectively output from the plural differential amplifiers, a determination circuit for determining the mean value of the output of the preamplifier circuit according to threshold voltage based upon the output of the output voltage holding circuit and outputting a selection signal and a selecting circuit for selecting one signal out of the output of the plural differential amplifiers based upon the selection signal.
An optical communication device according to the present invention is provided with an optical transmitting circuit for outputting an optical signal modulated based upon transmit data, an optical fiber transmission line for transmitting the optical signal and an optical receiving circuit according to a seventh aspect for receiving the optical signal output via the optical fiber transmission line.